If a wind-turbine tower is oscillating there will be the effect of tower-movements. This results to a big load, which acts on a mounted yaw-system of the wind-tower, on a gear within the wind-turbine nacelle and on the tower itself.
It is known to reduce this effect by a so called “active damping” of the tower oscillation.
The U.S. Pat. No. 7,309,930 B2 describes a solution to damp vibration based on a controlled torque. A vibration damping system for the wind-turbine is mounted on the tower. The vibration damping system comprises a vibration damper, which uses a variable signal to control the torque. The variable signal is based on a speed of a generator.
Another way to deal with tower oscillations is to design relevant mechanical constructions strong enough to tolerate extra forces, which are induced by the tower-movements.
Additionally relevant spear parts have to be changed more often because of wear and tear.